A study of more than 1,000 young Danish men suggests BMI alone may overlook reproductive differences associated with higher body fat.
Study: Body fat percentage in relation to semen characteristics, testicular volume, and reproductive hormones: a cross-sectional study. Image Credit: Lightspring / Shutterstock
A recent study published in the journal Human Reproduction found that a higher body fat percentage was associated with lower total sperm count and altered levels of reproductive hormones in young men. The association with sperm count was stronger among men with normal body mass index (BMI). However, body fat percentage was not associated with testicular volume.
Background
Obesity and overweight are currently estimated to affect up to a third of the world’s population. In parallel, sperm counts have been reported to be declining, while infertility rates are increasing. The use of assisted reproductive technologies (ART) is also on the rise.
Both overweight and obesity have been postulated to cause changes in reproductive hormones, oxidative stress, increased scrotal temperature, and epigenetic changes. BMI is among the most common measures of body weight.
Couples with men who have a high BMI are more likely to experience a longer time to pregnancy and have a lower live birth rate per ART cycle. High BMI is associated with poorer semen quality, including volume, sperm count, sperm concentration, vitality, motility, and morphology, as well as altered reproductive hormone levels.
It is unclear whether BMI or body fat percentage is a better predictor of semen characteristics, hormone levels, and testicular volume. BMI does not distinguish fat from lean mass. Body fat percentage may provide a more direct measure of adiposity than BMI or waist-to-height ratio.
However, body fat percentage has rarely been studied in relation to semen quality, testicular size, and reproductive hormones, and there is little clarity on whether high body fat is associated with male reproductive markers despite normal BMI.
Study Characteristics
The researchers in Denmark analyzed data from 1,058 Danish men aged approximately 19 years participating in the Fetal Programming of Semen Quality (FEPOS) cohort.
Body fat percentage was measured using bioelectrical impedance and classified as:
- Low: <8%
- Normal: 8–19% (reference group)
- High: 20–24%
- Very high: ≥25%
The investigators also measured BMI and waist-to-height ratio for comparison.
Participants provided semen and blood samples, self-measured their testicular volume, and completed questionnaires on health and lifestyle. The analyses accounted for parental education, maternal BMI and smoking, birth weight for gestational age, participants’ smoking, alcohol use, and physical activity, and relevant sample-collection factors.
Among the 978 men with complete information on all covariates, about 75% had normal body fat percentage, compared to 11% with high and 5% with very high values. The proportion with low fat was 9%. High and very high body fat percentages were linked to having parents with less education, mothers with higher BMI before pregnancy, and maternal smoking in the first trimester.
Men with higher body fat percentages also had higher birth weight for gestational age, BMI, and waist-to-height ratios. They smoked and drank alcohol less frequently, were less likely to exercise frequently, and their semen samples had a shorter ejaculation-to-analysis time than those with lower percentages.
Higher Body Fat Linked to Lower Total Sperm Count
Compared with men who had a normal body fat percentage, those with high body fat had a 23% lower total sperm count, while those with very high body fat (≥25%) also tended to have lower total sperm counts. The authors note that such reductions may be clinically relevant to the minority of men with borderline semen quality, despite the mean total sperm count being above the lower reference limit in the study population.
Estimates also pointed toward lower semen volume and sperm concentration, but they were imprecise and compatible with no association.
No clear associations were found between body fat percentage and sperm motility, sperm morphology, or testicular volume in the overall cohort.
Reproductive Hormones Altered
Increasing body fat percentage was associated with several changes in reproductive hormones, including:
- Lower testosterone
- Lower sex hormone-binding globulin (SHBG)
- Higher free androgen index
- Higher luteinizing hormone (LH)
- Higher estradiol
The free androgen index is calculated from testosterone and SHBG, and its increase likely reflects the lower SHBG levels in men with more body fat.
Men with low body fat generally showed hormonal changes in the opposite direction, although the estimates were inconsistent and compatible with no association.
Stronger Associations With Some Semen Measures Than BMI
This adiposity measure showed stronger associations with total sperm count, semen volume, and sperm concentration than either BMI or waist-to-height ratio. Associations with reproductive hormones were generally comparable across the three measures.
Notably, the association between higher body fat percentage and lower total sperm count was even stronger among men with normal BMI, while an association with a lower proportion of sperm with normal morphology emerged in this subgroup, although that estimate was imprecise. The reproductive hormone associations were weaker in this subgroup.
This suggests that men with relatively high body fat but normal BMI may still have reduced semen quality. The authors note the biological coherence of these observations, as “body fat percentage may better capture adiposity than BMI, a composite measure of body weight and height that is also affected by other factors such as muscle and bone mass.”
Conversely, the attenuated associations with hormone levels in normal-BMI men weaken the biological coherence of the subgroup findings, and the authors call for these findings to be replicated in future studies.
Limitations
The study had a relatively low participation rate (19%), raising the possibility of selection bias and limited generalizability, although the researchers used selection weights to reduce this risk. However, the authors note that the mean participant age of 19 years makes this less likely, as young adult men are rarely aware of their fertility potential.
Body fat percentage was measured by bioelectrical impedance, potentially introducing misclassification error because readings can vary with hydration, food intake, and recent exercise. The cross-sectional design precludes causal inferences. An indirect sensitivity analysis using BMI at age 11 yielded findings comparable to those obtained with adult BMI, reducing the likelihood of reverse causation.
Residual confounding and chance findings from the multiple comparisons remain possible. Self-measured testicular volume and the use of a single semen sample also introduced measurement uncertainty.
Conclusion
The findings suggest that higher body fat percentage is associated with lower total sperm count and altered reproductive hormone profiles in young men. The association with total sperm count was stronger among those with normal BMI, while most hormone associations weakened in this subgroup. Body fat percentage appeared to be a better indicator of some semen characteristics, particularly total sperm count, than BMI or waist-to-height ratio, while no association was observed with testicular volume.
These associations need to be explored in future prospective studies to clarify their temporal relationship and determine whether they are causal. Intervention studies would be needed to establish whether reducing excess body fat can improve male reproductive health.
